The present invention relates to a temperature sensing resistance element being preferably used as a component of a thermal flow sensor for measuring the flow rate of intake air in an internal combustion engine for an automobile, for example.
For example, it may sometimes be necessary to measure the flow rate of intake air for adjusting a fuel injection valve in an internal combustion engine for automobile ect. There is known, as a sensor for preforming such measurements (xe2x80x9cSensor Technologyxe2x80x9d page 25, September issue, 1989), a thermal flow sensor utilizing a resistor whose resistance value increases as temperature goes up (hereinafter referred to as a xe2x80x9ctemperature sensing resistance elementxe2x80x9d).
As the temperature sensing resistance element for a thermal flow sensor, there is known, for example, a resistance element of a structure comprising, as shown in FIG. 4, a cylindrical substrate 23 being made of an insulating material, a temperature sensing resistor 22 formed on the outer peripheral surface of the substrate 23 so as to connect both ends of the substrate 23, and a conducting member 24 such as a lead wire; the member being electrically connected to the temperature sensing resistor 22 with extending from both ends of the substrate 23 (See Japanese Utility Model Laid-Open No 56-96326. etc.
In the above-described resistance element, however, the element has problems that the electrical connections between the temperature sensing resistor and the lead wires become defective frequently since a satisfactory fixation of the lead wire is difficult, and that the lead wire is apt to slip off due to the influence of vibrations etc. during the practical use, even if the electrical connections are good during manufacturing.
In order to solve this problem, one may think out a temperature sensing resistance element in which both ends of the substrate is provided with cap-shaped conducting members and the lead wire is fixed to the conducting members.
However, because it is important for a temperature sensing resistance element for a thermal flow sensor to be able to respond quickly to temperature changes, it is necessary that the size of this temperature sensing resistance element be as small as possible, for example, the outside diameter be 1 mm and the length be within about 3 mm. Similarly, the above-described cap-shaped conducting member is also required to have a small size.
And in pushing the end surfaces of the substrate into this small cap-shaped conducting member, problems also arise: for example, the end surfaces cannot be completely pushed so as to make it contact intimately with the innermost recess of the conducting member; or, even if the end surfaces are pushed completely thereinto, the firm fixation cannot be achieved since there still remains some loose portion in the intermediate space between the conducting member and the base body. With these problems this type of a temperature sensing resistance element might be inferior in reliability to the above-described temperature sensing resistance element of conventional structure.
The present invention was made in view of the above-described situation, and the object of the present invention is to provide a temperature sensing resistance element having a higher reliability than the temperature sensing resistance element of conventional structure, and a thermal flow sensor excellent in reliability and measuring accuracy by using the temperature sensing resistance element.
According to the present invention, temperature sensing resistance element is provided, which comprises a substrate formed of an insulating material, a temperature sensing resistor formed on at least a part of the outer peripheral surface of this substrate so as to connect both ends of this substrate, a cap-shaped conducting member having an enlarged portion formed at a vicinity of an opening thereof and being electrically connected to the temperature sensing resistor and disposed at each end of this substrate, and a protective film for covering at least the above-described temperature sensing resistor, wherein a gap is defined by an inner surface of the enlarged portion and a part of the outer surface of the substrate, and the gap is widened gradually toward an end of the opening of the conducting member.
Also, according to the present invention, there is provided a temperature sensing resistance element, which comprises a cylindrical substrate made of an insulating material, a temperature sensing resistor formed on the outer peripheral surface of this substrate for connecting both ends of this substrate, a cap-shaped conducting member electrically connected to this temperature sensing resistor and disposed at each end of this substrate, and a protective film for covering at least the above-described temperature sensing resistor, wherein the radius of curvature of both end edges of said substrate is 0.05 mm, or more.
Furthermore, according to the present invention, there is provided a thermal flow sensor in which these temperature sensing resistance elements are built in a bridge circuit.